1. Field of the Invention
The present invention relates to a radio transmission system that controls transmission power so as to solve the far-near problem about transmission quality. In particular, the present invention relates to a transmission power control apparatus that properly maintains a target value of uplink transmission quality as a reference value for transmission power control, and to a radio base station having the transmission power control apparatus.
2. Description of the Related Art
The CDMA (Code Division Multiple Access) system features in secrecy and interference resistance. In addition, since a transmission power control technology that solves the far-near problem about transmission quality has been established, the CDMA system is being actively used in mobile communication systems and various radio transmission systems.
FIG. 9 is a schematic diagram showing an example of the structure of a radio transmission system that controls transmission power.
In the drawing, disposed in a radio zone 31 formed by a radio base station 30 are terminals 40-1 to 40-N. The radio base station 30 is connected to a base station control station 50 through a communication link 32.
The radio transmission system having this structure controls the transmission power in the following manner.
By referring to a target SIR value supplied by the base station control station 50 as will be described later, it performs the following processes.                The base station control station 50 monitors SIRs (Signal to Interference Ratios) of received waves that have arrived from the terminals 40-1 to 40-N through predetermined radio channels (for simplicity, assuming that they are radio channels that are allocated under a channel control or that are known under a zone structure and a frequency allocation) at predetermined intervals (for example, 800 times/second to 1500 times/second).        The base station control station 50 transmits to each of terminals as transmission ends of the received waves a transmission power command that contains binary information that means the transmission power of the received wave should be increased or decreased to a value of which the deviation of the SIR against the target SIR value is decreased.        
Thus, the transmission quality of the radio transmission channels formed between the radio base station 30 and each of the terminals 40-1 to 40-N is properly maintained free of unnecessarily interference and disturbance against other radio channels formed in parallel in accordance with the CDMA system even if characteristics of the radio transmission channels such as movement of terminals, variations of ground shapes, and variations of buildings widely and sharply vary as long as the target SIR value is a properly value.
To accomplish the transmission power control performed at high speed, the radio base station 30 and the terminals 40-1 to 40-N associatively operate. These associations are simply referred to as inner loops.
The radio base station 30 and the base station control station 50 associatively operate in parallel with the inner loops.                The radio base station 30 successively transfers both messages individually received from the terminals 40-1 to 40-N and their transmission quality (for simplicity, it is assumed that the transmission quality is supplied as error information) to the base station control station 50 through a communication link 32.        The base station control station 50 monitors the transferred transmission quality at intervals of a predetermined period (for simplicity, it is assumed that the period is 4 seconds to 5 seconds) and properly updates the target SIR value with a value so that proper transmission quality is obtained.        The radio base station 30 applies the target SIR value to the inner loops.        
Association of the radio base station 30 and the base station control station 50 is simply referred to as the outer loop.
In the foregoing related art reference, a trigger at which the target SIR value is updated is decided regardless of variations of characteristics of the radio transmission channels. Thus, in the related art reference, transmission power is not always properly controlled because of the following reasons.                The target SIR value that is updated is largely delayed (FIGS. 10(3) and (4)) after a trigger at which the transmission quality (error rate) is largely deteriorated (FIG. 10(1)) or improved (FIG. 10(2)). Thus, in any of the terminals 40-1 to 40-N, the transmission power is maintained at an excessively high value or the uplink transmission quality temporarily deteriorates.        Even if the deterioration (FIG. 10(b)(1)) of the transmission quality (error rate) of a message is very low, the transmission power may be set to an excessively high value (FIG. 10(b)(2)).        The transmission power that is updated is largely delayed after a trigger at which the SIR of the message deteriorated or improved (FIG. 10(c)(1), (2)). Thus, transmission power cannot be controlled in real time. In addition, the target SIR value cannot be always properly set.[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-274748 (claim 1)        